In the conventionally used pass-monitoring apparatus of the optical network system, pass-monitoring is carried out as follows. In a transmitting station, pass-monitoring signals are superposed on a wavelength division multiplexed (WDM, hereinafter) optical signal, which is propagated through an optical transmission line, and the pass-monitoring signals are monitored in a receiving station. For example, in the pass-monitoring apparatus of the optical network system disclo ed in Japanese Patent Kokai 7-193558, the pass-monitoring signals for identifying respective signal wavelengths are multiplexed and collectively superposed on a WDM optical signals and the pass-monitoring signals are monitored regarding the respective signal wavelengths in the receiving station.
In the aforementioned apparatus, pass-monitoring is carried out between the transmitting and receiving stations via the optical transmission lines. Accordingly, when the matrix switch is inserted in the optical transmission lines, there is no means for discriminating whether monitoring of the optical transmission lines and an operation of the matrix switch normally operate or not, because the optical transmission lines are interchanged in the receiving station. Moreover, if fails arise in the optical transmission line or the matrix switch, there is no means for locating the fails.